Can I play the DLC from the point where I completed the main game story-line, or do I have to back up to some save-game that I created before I entered the final battle?
There is another solution, check out this mod. It worked when I tried it, it's worth a shot.
Simply download it, and place it in your
Data folder where you installed Fallout NV, e.g
C:\Program Files (x86)\Steam\steamapps\common\fallout new vegas\Data. Then start the game laucher, and tick of the mod in